The invention relates to a steering funnel for bed material in a fluidized bed boiler, a fluidized bed boiler and a method in a fluidized bed boiler. The invention especially relates to a new method for guiding flows of material to be removed from a fluidized bed boiler.
An ordinary fluidized bed boiler essentially comprises a furnace limited by vertical walls, a bottom and a roof. A fluidized bed formed of solid matter particles like sand is maintained in the furnace. A typical fluidized bed boiler comprises feeding means for feeding fuel, fluidized bed material and air to the furnace. Flue gases are led away from the furnace to an exhaust duct for flue gases. At the bottom of the furnace there is a grate in which feeding elements of air, like air nozzles, are typically arranged. Air needed for burning of fuel and for fluidization of bed material is fed from air nozzles to the bottom of the furnace. Fluidization air is typically used as primary air of combustion process. On the grate are also arranged elements like openings or gaps for removing solid matter, i.e. bed material, ash, and impurities among them, to underside of the grate. Below the openings or gaps of the grate is typically arranged one or more channels or pockets, which are in this text called funnels, along which material to be removed from the furnace is led in a controlled manner from the furnace to the underside of the boiler. There usually are shutters on the bottom end of the funnels, by means of which the volume of the material to be removed from the furnace is controlled. Material to be removed from the funnels is usually led to a conveyor and onwards to further processing. Material is removed from the funnels at the need, typically periodically, for example a few times an hour in a normal operating situation.
On the underside of the grate there can be a windbox, inside which said funnels are at least partially arranged. Fluidization air is led to the windbox, i.e. around funnels, and fluidization air is led from the windbox to the air feeding elements above the grate. Typically air is heated in the windbox with the heat of bed material streaming downwards in the funnels. At the same time, bed material in the funnels is cooling.
One problem in a fluidized bed boiler is a high proportion of incombustible particles contained in some fuels, such as construction waste or logging waste, such as stumps: metals, stones, concrete, etc., that accumulate on the bottom of the furnace and interfere with fluidization of sand. The above-mentioned problem has been solved by designing the bottom of the furnace to better remove bed material, for example by enlarging discharge openings of material that lead through the grate. In this manner, also impurities at the bottom of the furnace are removed from the furnace with the bed material.
Due to the downwards tapering form of one or more funnels below the grate, the flow in the funnel is not always steady. A so-called core flow usually tends to develop at the discharge opening in the bottom of the funnel or to the center of the funnel. This means that material normally flows faster downwards at the center of the funnel than at the edges of the funnel. Because in fluidized bed boilers the cross-sectional area of the bottom of the furnace, i.e. the bed, is typically considerably larger than the area of discharge openings at the bottom of the funnel or funnels, material is removed irregularly from the furnace. Sometimes material is removed in fluidized bed boilers only from said core flow area of the bottom of the furnace.
As a result of irregular discharge of material, unburnt material accumulates to piles in the bottom of the bed hindering fluidization, and it may cause a partial or total melting of bed material.
Because of an irregular flow of material, a part of the material may even be permanently stationary or move very slowly on the edges of the funnels. This disturbs heat transfer between air flowing in the windbox and bed material. In this case, the material to be removed from the funnels, having flown in the core flow, does not have enough time to cool down as desired in the funnel. On the other hand, air blown to the furnace is not pre-heated in a desired manner in the wind box.